The present invention relates generally to the vapor exhaust system of a supercooled rotor and, more specifically, to a vapor exhaust system with a vapor trap associated in cooperation with a vapor pump within the central bore of a torque tube of a superconducting rotor.
In the supercooled rotor of a superconducting generator, superconducting field coils are immersed in a pool of liquid coolant, such as helium. As heat is removed from the superconducting field coils, the liquid coolant is caused to boil off at a generally constant rate. The resulting helium vapors are removed from the helium reservoir of the rotor through an exhaust pipe. The helium vapor passes through the exhaust pipe and is conducted out of the supercooled rotor to a refrigeration system where it is eventually condensed back into a liquid form for reintroduction into the supercooled rotor. A vapor pump, connected in fluid communication with the exhaust pipe, is utilized to encourage this flow of helium vapor away from the helium reservoir.
The vapor pump comprises a portion of the exhaust pipe which extends in a radial direction away from the central axis of the superconducting rotor. As the rotor revolves about this central axis, the helium vapor is caused to flow through the vapor pump by the resulting centrifugal forces acting on it. The efficiency of the vapor pump is a function of the density of the helium gas, the angular velocity of the vapor pump about the central axis of the rotor and the effective radial distance between the inlet and outlet portions of the vapor pump.
In typical applications, the vapor pump of a supercooled rotor, which comprises a section of the rotor's vapor exhaust pipe, is disposed between the helium reservoir and a portion of the exhaust pipe which is disposed in contact with a torque tube. This contact of the exhaust pipe and the torque tube allows the helium vapor to flow in thermal communication with the torque tube and to thus reduce the temperature of the torque tube and also reduce the heat which is being conducted toward the cold zone of the rotor. The portion of the exhaust pipe which is in contact with the torque tube is generally provided with a plurality of baffles which inhibit convection currents from forming within that portion of the exhaust pipe. These convection currents, if permitted to form, would reduce the efficiency of the vapor cooling of the torque tube and significantly increase the rate of heat flow through the torque tube.
Since the cold end of the torque tube is at a higher temperature than the helium vapor passing through the exhaust pipe, it is possible for the helium vapor to be warmed as it exits from the vapor pump and be induced to reverse its direction of flow within the exhaust pipe. Even though the gas experiences very high centrifugal forces which tend to cause it to flow in a radially outward direction, this warmed helium vapor could then possibly flow in a radially inward direction, past outwardly flowing colder helium, through the vapor pump and reenter the helium reservoir. If the diameter of the exhaust pipe is relatively small, this reverse flow of warmed gas is inhibited by the reluctance of the helium vapor to flow in two directions simultaneously through the vapor pump. This reluctance is caused by frictional forces and mixing between these two counter flowing streams of vapor. However, if the exhaust pipe is of significant diameter as may be required for rapid cool down procedures, separate streams of warm and cold helium vapor can flow in opposite directions through the vapor pump.
In a rotor which is rotating about its central axis, the temperature of a vapor can determine its direction of flow. Relatively cold vapor, due to its higher density, tends to flow in a radially outward direction, whereas warmer vapor, due to its lower density, tends to flow in a radially inward direction. Since thermal communication with the torque tube can significantly raise the temperature of helium vapor passing through the exhaust pipe, a potential reverse flow through the exhaust pipe can be created.
The torque tube is approximately 10.degree.-20.degree. Kelvin in the region where the exhaust pipe is in contact with it and the helium gas flowing in the normal direction through the exhaust pipe is slightly warmer than the liquid helium pool within the helium reservoir which is approximately 4.degree. Kelvin. Therefore, the helium vapor can experience a temperature increase as it passes in thermal communication with the warmer torque tube in the region where the exhaust pipe and the torque tube are first in contact and in thermal communication with each other. The warmed helium vapor can then tend to flow in a radially inward direction through the vapor pump portion of the exhaust pipe.
If warmed helium vapor is permitted to flow in a reverse direction through the exhaust pipe of a supercooled rotor, the radially central portion of the helium reservoir can be eventually backfilled with unacceptably warm helium. When this condition exists, the helium reservoir contains liquid helium along its radially outward boundary with a cylindrically shaped quantity of warm helium vapor disposed about its central axis. Between the liquid helium and the warm helium vapor, a cylindrically shaped region of colder helium gas will exist. As more warm helium vapor enters the helium reservoir in a reverse direction through the exhaust pipe, this central cylinder of warm vapor increases in radius until the mouth of the exhaust pipe, which is in fluid communication with the helium reservoir, is completely covered with warm helium gas. When this condition occurs, only warm helium vapor is available to the inlet mouth of the exhaust pipe and the efficiency of the vapor pump is severely decreased and disadvantageous thermal oscillations can be created within the cooling circuit.
The present invention incorporates a generally U-shaped vapor trap within the exhaust pipe of a supercooled rotor. The vapor trap is disposed in fluid communication with the exhaust pipe between the helium reservoir and a portion of the exhaust pipe which is in contact with the torque tube. The vapor trap comprises a generally U-shaped section of pipe with two generally straight legs extending from the U-shaped section in a direction toward the central axis of the rotor. A first leg is disposed in a direction toward the portion of the exhaust pipe which is connected in direct fluid communication with the helium reservoir and a second leg is disposed in a direction generally toward the portion of the exhaust pipe which is in contact with the torque tube. The first leg which extends toward the helium reservoir is made longer than the second leg in order to improve the efficiency of the vapor trap and the operation of the vapor pump.
Since colder vapor tends to flow in a direction away from the central axis of a rotating member, the U-shaped portion of the present invention provides a collecting region for a volume of cold helium vapor within the radially outward portion of the vapor trap. If warmed helium vapor attempts to flow in a radially inward direction from the region of the exhaust pipe which is in contact with the torque tube, it will encounter this volume of colder helium gas within the vapor trap and thus the radially inward flow of warmed helium gas will be discouraged.
The present invention can comprise a U-shaped portion of the exhaust pipe itself or a separate component connected in fluid communication with the exhaust pipe between the helium reservoir and the portion of the exhaust pipe which is disposed in contact with the torque tube. By collecting a portion of relatively colder helium gas within the vapor trap, the flow of warmer helium gas in a reverse direction through the exhaust pipe is inhibited.